The present invention relates to an apparatus for detecting the presence of a magnetic tag and more particularly to a hand-held detector operated by passing the detector through a region proximate to an article in which a tag may be disposed out of plain view.
It is known to provide identification tags, such as coded-wire tags, for organisms. The tags can be implanted in an organism for purposes of identification of the specimen involved. Such tags can be used for the identification of fish, in which case the tags are implanted at any one of a number of locations below the skin layer of the fish. Such fish tags can be very small, on the order of 0.25 mm diameter by 1.05 mm length. The tags can be segments of stainless steel wire which can have some code to be used as an identification for the specimen involved. These tags can be used to collect information regarding such things as fishery management when the tags are implanted in the fish.
Recovery of these small tags usually involves detection of the magnetic nature of the tags. Because of the size of the tags and the biological constraints of the materials used for tags, this has been a difficult proposition. In particular, the biological constraints prevent the use of the best of magnetic materials with the result being that typically tags have a magnetic moment, .mu..sub.0 M, of about 10.sup.-11 Weber-meters. Such a tag thus produces a magnetic field B at a useful distance from the tag R of approximately four inches (approximately 0.1 meters) of B=.mu..sub.0 M/4.pi.R.sup.3 =8.times.10.sup.-10 Tesla. For comparison purposes, the magnetic field of the Earth is approximately B.sub.E =6.times.10.sup.-5 T. Thus, the magnetic field produced by the tag at a distance of approximately four inches from the tag is approximately 1/75,000 of the local field due to the Earth at the same point.
It has been known to provide detectors of various geometries to successfully and routinely detect these tags despite the small magnetic field produced by the tags. These detectors operate by detecting a tiny change due to the tag in the much larger but steady local magnetic field that is produced by the Earth. The tiny change in the field is the result of moving the organism which contains the tag, and thus moving the tag, through a sensitive volume of a stationary detector. These detectors are all sensitive only to changes in the local magnetic field. Since the detector does not move, the local field essentially does not change. However, an article passing through the local magnetic field, producing its own magnetic field, changes the local magnetic field. The detector then indicates the presence of such a tag when the detected local magnetic field varies. Furthermore, such stationary detectors also use various combinations of shielding and cancellation to reduce the effects of both natural and man-made variations in the local magnetic field to acceptable levels.
Despite the success of such stationary detectors, there is a need for detectors which avoid the requirement of moving the specimen with respect to the detector. It is desirable to leave the specimen of interest stationary and move a detector over the appropriate part of the specimen. However, this provides a much more difficult problem.
Essentially, an instrument intended to accomplish this must measure the magnetic field as close as possible to the volume of interest and then subtract from that value the value of the magnetic field a short distance away. Such an instrument is called a differential magnetometer or gradiometer. Generally, gradiometers are well known. They are usually implemented with two or more static magnetometers of one of several varieties such as flux-gate magnetometers, optically pumped atomic magnetometers, nuclear magnetic residence magnetometers (free-precession magnetometers), or SQUID (superconducting quantum interference device) magnetometers. However, all of these methods are inappropriate for the task at hand because of their relative complexity, their cost, size or lack of portability.